The present invention relates to nonlinear optics and, more particularly, to a method using a monolithic monocrystalline material for producing radiation of a selected frequency by quasi-phase-matching, a monolithic monocrystalline material which optimizes nonlinear interactions by quasi-phase-matching, and a method of fabricating the same. It also relates to an optical generator utilizing such material.
The properties of nonlinear materials have many different applications; the invention uses these properties for frequency conversion. That is, nonlinear monocrystalline materials can interact with input radiation to generate output radiation of a frequency different than that of the input radiation. The input and output radiation typically is optical radiation, i.e., radiation having frequencies in the visible range or of other adjacent frequency ranges such as ultraviolet, and infrared. It encompasses the ultraviolet, visible, and near, mid and far infrared frequencies. (The term "crystalline" as used herein without a modifier means "monocrystalline" unless the context indicates otherwise.)
Second harmonic generation is an example of one of the interactions possible in a nonlinear crystalline material. In second harmonic generation, the frequency (.omega.) of input optical radiation is converted into optical radiation of twice such frequency (2 .omega.). A sum frequency generator also relies on frequency conversion in a nonlinear crystalline material. In such an arrangement, input beams of two different frequencies (.omega..sub.1 and .omega..sub.2) are summed by a nonlinear material to generate optical radiation whose frequency (.omega..sub.3) is the sum of the frequencies of the two input beams (.omega..sub.1 +.omega..sub.2 =.omega..sub.3). Other common devices utilizing a nonlinear monocrystalline material are optical parametric oscillators, optical parametric amplifiers and difference frequency generators.
Nonlinear frequency conversion in crystalline materials is used to provide a desired frequency which is not conveniently available from an existing source such as an existing laser. For example, it is used to "frequency double" the 10.6 .mu.m radiation (approximately 30 THZ) generated by a CO.sub.2 laser, to 5.3 .mu.m (approximately 60 THZ). Currently there is no efficient or high power laser capable of generating the latter frequency. This frequency, however, is highly desirable because the atmosphere is transparent in the 3-7 .mu.m spectral range. Application examples are remote sensing, laser radar and laser countermeasure systems.
Efficient nonlinear frequency conversion requires phase matching as will be described in some detail below. Because the frequency range in conventionally phase-matched crystals is limited by the birefringence available, consideration has been given to quasi-phase-matching as also described below. Reference is made, for example, to the papers entitled "Interactions Between Light Waves in a Nonlinear Dielectric" by J. A. Armstrong et al., Physical Review, Vol. 127, 1962, pp. 1918-1939; "Optical Harmonics and Nonlinear Phenomena" by A. Franken et al., Review of Modern Physics, Vol. 35, 1963, pp. 23-39; "A Quasi-Phase-Matching Technique for Efficient Optical Mixing and Frequency Doubling" by A. Szilagyi et al., J. Appl. Physics, Vol. 47, No. 5, May 1976, pp. 2025-2032; and "Second-Harmonic Generation in GaAs `Stack of Plates` Using High-Power CO.sub.2 Laser Radiation" by D. E. Thompson et al., Appl. Phys. Lett., Vol. 29, No. 2, 15 Jul. 1976, pp. 113-115.
Quasi-phase-matching has been successfully achieved in ferroelectric nonlinear crystals in which domains can be inverted. (See "Quasi Phase Matched Second Harmonic Generation of Blue Light in Periodically Poled LiNbO.sub.3 " by G. A. Magel et al., Appl. Phys. Lett., Vol. 56, 1990, pp. 108-110; and "Second Harmonic Generation of Green Light in Periodically Poled Planar Lithium Niobate Waveguide" by E. J. Lim et al., Electronic Letters, Vol. 25, 1989, pp. 174-175.) While this approach has been widely used for manufacturing waveguides in lithium niobate and other poleable materials, it is limited in that the technique is not possible in non-poleable materials. That is, a different approach is necessary to alternate the sign of the optical nonlinear coefficient in a non-poleable crystal.
Earlier approaches that have been considered for non-poleable materials required a multitude of thin slices of crystal. (See "A Quasi-Phase-Matching Technique for Efficient Optical Mixing and Frequency Doubling" by A. Szilagyi et al., J. Appl. Physics, Vol. 47, No. 5, May 1976, pp. 2025-2032; and "Second-Harmonic Generation in GaAs `Stack of Plates` Using High-Power CO.sub.2 Laser Radiation" by D. E. Thompson et al., Appl. Phys. Lett., Vol. 29, No. 2, 15 Jul. 1976, pp. 113-115.) While good agreement was found between theory and experiment, the work was not continued for several reasons, including excessive reflection, scattering, and absorption losses caused by dust, surface quality and alignment tolerances. Thus, the number of air/material interfaces made this technique impractical. Because of this lack of practicality, those in the art have generally abandoned quasi-phase-matching as a general approach to bulk nonlinear conversion in non-poleable materials.